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recombination processes and polaron effect in perovskite manganite thin films
Physica B 284}288 (2000) 1912}1913
Carrier generation/recombination processes and polaron e!ect in perovskite manganite thin "lms G.-R. Wu , Masasi Inoue , Minoru Sasaki *, Hiroshi Negishi , G.-C. Xiong Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8526, Japan National Mesoscopic Physics Laboratory, Peking University, Beijing 100871, People's Republic of China
Abstract Pulsed-laser-induced transient thermoelectric e!ect (TTE) for perovskite manganite "lms, La Ca MnO (LCMO), Pr Sr MnO (PSMO), La Sr MnO (LSMO), has been measured under DC electric and magnetic "elds. The photoinduced TTE voltage and its sign are strongly dependent on both temperature and magnetic "eld, which is due to the formation of a bipolar state of photogenerated electrons and holes. It is found that Jahn}Teller and polaron e!ects are appreciable near the Curie temperature for the carrier recombination, in particular, for LCMO with small ionic radius of Ca> ions. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Polaron e!ect; Recombination process; Transient thermoelectric e!ect
Extensive studies have currently been made on perovskite manganites, R A MnO (R: trivalent rare earth, \V V A: divalent alkaline-earth), that exhibit a colossal magnetoresistance (CMR) [1}3]. These materials have the following features: (i) It contains a mixed valent Mn> (t e ) and Mn> (t ) ions and two neighboring locali zed Mn spins are coupled by a double-exchange (DE) interaction through the e originated carriers (holes), leading to a ferromagnetic ordering below a Curie temperature ¹ . (ii) Jahn}Teller (JT) and polaron e!ects are now known theoretically to play a crucial role in CMR [4], and thus the original Mn> and Mn> derived e bands are modi"ed by both DE and JT interactions [5]. In order to study the transport properties of these materials, we have made a DC transport as well as dynamic measurements, called pulsed-laser-induced `transient thermoelectric e!ecta (TTE) [6}8], for three perovskite manganite "lms, La Ca MnO (LCMO), Pr Sr MnO (PSMO), La Sr MnO (LSMO), in their paramagnetic (P) and ferromagnetic (FM) phases. Thin "lms were grown on a (1 0 0)-LaAlO single crystal * Corresponding author. E-mail address: [email protected] (M. Sasaki)
by a laser ablation method. The TTE measurements were made using Nd : YAG laser; experimental details were described elsewhere [6}8]. An external DC voltage < up to 25 V was applied to the samples (< N c-axis) "! "! and a magnetic "eld B perpendicular to the "lm (B""caxis) using a superconducting magnet up to 5 T. The temperature dependence of DC resistivities of all "lms with and without magnetic "eld, showing CMR, are in general agreement with the reported data [9], though not shown here. TTE voltage is quite small when < "0, but is en"! hanced appreciably upon application of DC voltage. Typical TTE signals at < "25 V for LCMO in FM "! and P phases are shown in Fig. 1. With increasing temperature, the peak intensity is decreased and its sign is reversed near the Curie temperature (¹ "273 K) from positive to negative (sign reversal), from which we can determine the `reversal temperaturea, ¹ . Small oscilla tory behaviors are not concerned here. When the DC voltage is reversed, the TTE voltage is also reversed symmetrically from positive to negative. The TTE decay curve < can be "tted by the multiple exponential form as, <"< #Ra exp (!t/q ), where G G < is an in"nite value at tPR, a a relaxation ampli G tude, and q a relaxation time for the ith decay process; (i) G
0921-4526/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 3 0 1 1 - 2
G.-R. Wu et al. / Physica B 284}288 (2000) 1912}1913
1913
Fig. 2. Temperature dependence of the evaluated capture cross section using the observed relaxation time q for two "lms in logarithmic scales.
Fig. 1. Typical TTE pro"les of LCMO in the FM (above) and P phases (below).
`stage 1a due to a carrier recombination of photogenerated electron}hole pairs and/or (2) `stage 2a due to a thermal di!usion of conduction carriers along a concentration gradient [6}8]. The data analysis reveals that the former process is valid in the present case. As marked in Fig. 1, the TTE pro"le is characterized by two relaxation times q and q for the recombination processes via two di!erent capture centers. Using the well-known expression for a carrier capture cross-section p"1/(q v N) with the observed relaxation time q , we have evaluated p for two samples, as given in Fig. 2. p obeys the power law of the form, pJ¹\ far below ¹ , while near ¹ it deviates from the power law. In usual semiconductors, the capture cross section for neutral impurity centers is temperature independent and that for ionized impurities is pJ¹\. The deviation
near ¹ is due to JT and polaron e!ects, which are appreciable for LCMO compared to PSMO, since the ionic radius of Ca> is smaller and thus its JT distortion is larger than those of Sr> ions. In conclusion, from the dynamic TTE experiments we have found that the JT and polaron e!ects are predominant near the ferromagnetic Curie temperature for the carrier recombination process in these CMR materials.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R.M. Kuster et al., Physica B 155 (1989) 362. K. Chabara et al., Phys. Rev. Lett. 71 (1993) 1990. R. von Holmet et al., Phys. Rev. Lett. 71 (1993) 2331. A.J. Millis et al., Phys. Rev. Lett. 74 (1995) 5144. Y. Moritomo et al., Phys. Rev. B 56 (1997) 5088. M. Sasaki et al., J. Appl. Phys. 59 (1986) 796. M. Sasaki et al., Phys. Rev. B 46 (1992) 1138. M. Sasaki et al., J. Appl. Phys. 81 (1997) 7817. G.C. Xiong et al., Appl. Phys. Lett. 66 (1995) 1689.
Carrier generation/recombination processes and polaron e!ect in perovskite manganite thin "lms G.-R. Wu , Masasi Inoue , Minoru Sasaki *, Hiroshi Negishi , G.-C. Xiong Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8526, Japan National Mesoscopic Physics Laboratory, Peking University, Beijing 100871, People's Republic of China
Abstract Pulsed-laser-induced transient thermoelectric e!ect (TTE) for perovskite manganite "lms, La Ca MnO (LCMO), Pr Sr MnO (PSMO), La Sr MnO (LSMO), has been measured under DC electric and magnetic "elds. The photoinduced TTE voltage and its sign are strongly dependent on both temperature and magnetic "eld, which is due to the formation of a bipolar state of photogenerated electrons and holes. It is found that Jahn}Teller and polaron e!ects are appreciable near the Curie temperature for the carrier recombination, in particular, for LCMO with small ionic radius of Ca> ions. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Polaron e!ect; Recombination process; Transient thermoelectric e!ect
Extensive studies have currently been made on perovskite manganites, R A MnO (R: trivalent rare earth, \V V A: divalent alkaline-earth), that exhibit a colossal magnetoresistance (CMR) [1}3]. These materials have the following features: (i) It contains a mixed valent Mn> (t e ) and Mn> (t ) ions and two neighboring locali zed Mn spins are coupled by a double-exchange (DE) interaction through the e originated carriers (holes), leading to a ferromagnetic ordering below a Curie temperature ¹ . (ii) Jahn}Teller (JT) and polaron e!ects are now known theoretically to play a crucial role in CMR [4], and thus the original Mn> and Mn> derived e bands are modi"ed by both DE and JT interactions [5]. In order to study the transport properties of these materials, we have made a DC transport as well as dynamic measurements, called pulsed-laser-induced `transient thermoelectric e!ecta (TTE) [6}8], for three perovskite manganite "lms, La Ca MnO (LCMO), Pr Sr MnO (PSMO), La Sr MnO (LSMO), in their paramagnetic (P) and ferromagnetic (FM) phases. Thin "lms were grown on a (1 0 0)-LaAlO single crystal * Corresponding author. E-mail address: [email protected] (M. Sasaki)
by a laser ablation method. The TTE measurements were made using Nd : YAG laser; experimental details were described elsewhere [6}8]. An external DC voltage < up to 25 V was applied to the samples (< N c-axis) "! "! and a magnetic "eld B perpendicular to the "lm (B""caxis) using a superconducting magnet up to 5 T. The temperature dependence of DC resistivities of all "lms with and without magnetic "eld, showing CMR, are in general agreement with the reported data [9], though not shown here. TTE voltage is quite small when < "0, but is en"! hanced appreciably upon application of DC voltage. Typical TTE signals at < "25 V for LCMO in FM "! and P phases are shown in Fig. 1. With increasing temperature, the peak intensity is decreased and its sign is reversed near the Curie temperature (¹ "273 K) from positive to negative (sign reversal), from which we can determine the `reversal temperaturea, ¹ . Small oscilla tory behaviors are not concerned here. When the DC voltage is reversed, the TTE voltage is also reversed symmetrically from positive to negative. The TTE decay curve < can be "tted by the multiple exponential form as, <"< #Ra exp (!t/q ), where G G < is an in"nite value at tPR, a a relaxation ampli G tude, and q a relaxation time for the ith decay process; (i) G
0921-4526/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 3 0 1 1 - 2
G.-R. Wu et al. / Physica B 284}288 (2000) 1912}1913
1913
Fig. 2. Temperature dependence of the evaluated capture cross section using the observed relaxation time q for two "lms in logarithmic scales.
Fig. 1. Typical TTE pro"les of LCMO in the FM (above) and P phases (below).
`stage 1a due to a carrier recombination of photogenerated electron}hole pairs and/or (2) `stage 2a due to a thermal di!usion of conduction carriers along a concentration gradient [6}8]. The data analysis reveals that the former process is valid in the present case. As marked in Fig. 1, the TTE pro"le is characterized by two relaxation times q and q for the recombination processes via two di!erent capture centers. Using the well-known expression for a carrier capture cross-section p"1/(q v N) with the observed relaxation time q , we have evaluated p for two samples, as given in Fig. 2. p obeys the power law of the form, pJ¹\ far below ¹ , while near ¹ it deviates from the power law. In usual semiconductors, the capture cross section for neutral impurity centers is temperature independent and that for ionized impurities is pJ¹\. The deviation
near ¹ is due to JT and polaron e!ects, which are appreciable for LCMO compared to PSMO, since the ionic radius of Ca> is smaller and thus its JT distortion is larger than those of Sr> ions. In conclusion, from the dynamic TTE experiments we have found that the JT and polaron e!ects are predominant near the ferromagnetic Curie temperature for the carrier recombination process in these CMR materials.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R.M. Kuster et al., Physica B 155 (1989) 362. K. Chabara et al., Phys. Rev. Lett. 71 (1993) 1990. R. von Holmet et al., Phys. Rev. Lett. 71 (1993) 2331. A.J. Millis et al., Phys. Rev. Lett. 74 (1995) 5144. Y. Moritomo et al., Phys. Rev. B 56 (1997) 5088. M. Sasaki et al., J. Appl. Phys. 59 (1986) 796. M. Sasaki et al., Phys. Rev. B 46 (1992) 1138. M. Sasaki et al., J. Appl. Phys. 81 (1997) 7817. G.C. Xiong et al., Appl. Phys. Lett. 66 (1995) 1689.